Pressure sensors have applications in air movement control, fluid level measurement, leak detection, medical instruments, industrial process controls and environmental controls, particularly including heating, ventilating and air conditioning. In many of these applications, pressure difference measurements generated by sensors are indirect indicators of fluid velocity or fluid volume rate of flow. Application of such sensors in systems characterized by small pressure differences has proven both difficult or excessively expensive. Certain types of inexpensive pressure sensors, such as silicon diaphragm sensors, have sufficient sensitivity to resolve pressure changes of less than 1/40 of a pound per square inch but give unreliable performance in the field because of drift problems associated with instantaneous temperature, temperature cycling, aging of the silicon crystal and other factors.
Instructive to the problems in low pressure systems are the difficulties encountered in using silicon diaphragm sensors in heating, ventilating and air conditioning systems. Pressure transducers can be used in the outlet of a conduit into a room to generate a signal indicating the quantity of air being admitted into a room through the conduit.
Silicon diaphragm pressure sensors have failed to provide predictable behavior at the pressure differentials present in forced air distribution systems despite having adequate sensitivity for the application. Silicon diaphragm sensors are manufactured using techniques common to the integrated circuit industry and are relatively cheap and plentiful. Accordingly, their application to air circulation systems is desirable. A thin, deformable silicon diaphragm separates two differing pressure regimes, in a differential sensor one side being at the local ambient pressure and the other side being at a slightly altered pressure. Incorporated in the deformable sensor are piezoresistive circuit elements, arranged in a wheatstone bridge, the resistivity of which is a function of the extent of deformation of the diaphragm.
Silicon diaphragm pressure sensors are typically designed to operate with full scale pressure of about 1.0 p.s.i. Application of the sensors to determining conduit air pressure typically entails operation over a small fraction of this range. Such operation exaggerates the drift problems exhibited by the sensors due to aging and thermal hysteresis. The sensors also exhibit considerable mechanical variability from sensor to sensor and are sensitive to changes in ambient temperature. All of these factors combine to produce considerable drift in the signal generated by the sensors.